1. Field of the Invention
This invention relates to a method and apparatus for reducing the mouth of a tubular body and more particularly but not exclusively to the formation of a shoulder and neck on a container body.
Cans for soft drinks are provided with a shoulder neck and flange of diameter less than that of the rest of the can body in order to reduce the diameter of the mouth so that a smaller, and therefore cheaper, aluminium or steel tear-open can end may be used. Aerosol containers are also known which have a shoulder portion terminating in an aperture to receive the standardised 1 inch diameter valve cup.
2. Description of Related Art
In known die necking operations, such as are described in U.S. Pat. No. 1,698,999 (Hothersall) or U.S. Pat. No. 4,527,412 (Stoffel et al), each die is only able to form a limited amount of reduction in the diameter of the tubular body presented to it. Typical reductions in diameter at each die are discussed in U.S. Pat. No. 3,995,572 (Saunders) as being about 3.1 mm (0.122 inch) on a steel body 38.1 mm (1.5 inch) dia. at a first die and lesser reductions at subsequent dies. Whilst greater reductions can be achieved by a combination of a die necking process followed by a roll forming operation as is discussed in British Pat. No. 2,083,382 (Metal Box) the necessity for both die necking and roll forming apparatus incurs considerable capital expense.
In the tube drawing art several authors have proposed the use of ultrasonic vibration of the plug or die to assist the reduction process. In U.S. Pat. No. 3,212,312 (Boyd), apparatus is described in which both plug and die are vibrated in a direction along the axis of the tube as the tube is drawn between the plug and die to be reduced in diameter. Boyd's claims relate to a drawbench in which a selected phase relationship is deliberately established between the vibration at the plug and at the die. British Pat. No. 1,379,234 (Dawson & Sansome) also describes a draw bench in which both plug and die are vibrated axially and concludes that optimum performance is achieved in certain cases when the phase angle between the plug and die vibrations is neither 0.degree. nor 180.degree. but is somewhere between the two, the value depending on several factors.
British Pat. No. 1,389,214 (Sansome et al) describes apparatus for drawing a circular metal blank into a cup. The apparatus comprises a die, a punch and a blank holder. The die and blank holder are connected to an array of transducers which induce radial vibrations in the die and blank holder to assist drawing of the blank as the punch enters the die.
This prior art of draw benches illustrates that benefit of reduced friction at the interface between the workpiece and die can be achieved by ultrasonic vibration of the die but does not teach tool arrangements suitable for localised deformation of one end portion of a tubular body such as a can body. In an article in SOVIET ENGINEERING RESEARCH Vol. 3 No. 12 pages 54.55 titled "Upsetting of Aerosol containers using Ultrasound", apparatus is depicted and described in which a cylindrical container body 45 mm diameter is reduced by a sequence of several dies to 26.5 mm diameter to define a shoulder and neck to which an aerosol valve cup may be crimped. The apparatus comprises a short cylindrical plug entered into a die which has three transducers equispaced around it to induce radial vibrations in the die. We have found that the cylindrical plug is not ideal because the tubular body is not supported internally during the early part of the deformation process.